1. Technical Field
The present invention relates to magnetic recording media, and more particularly to magnetic recording media whose magnetic layer contain polymers having quaternary ammonium moieties and amine moieties. The present invention also relates to such polymers themselves and to a quaternary ammonium isocyanate compound and an amine isocyanate compound used to produce the polymers.
2. Background Information
Magnetic recording media generally comprise a magnetic layer coated onto at least one side of a non-magnetizable substrate. For particulate magnetic recording media, the magnetic layer comprises a magnetic pigment dispersed in a polymeric binder. The magnetic layer may also include other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
The polymeric binders of the magnetic layer are commonly derived from polymers which require curing in order to provide magnetic recording media with appropriate physical and electromagnetic properties. To prepare such media, the components of the magnetic layer are combined with a suitable solvent and thoroughly mixed to form a homogeneous dispersion. The resulting dispersion is then coated onto the non-magnetizable substrate, after which the coating is dried, calendared if desired, and then cured.
The polymeric binders of magnetic recording media are most commonly prepared from polymer blends comprising a hard component, i.e., a polymer with relatively high glass transition temperature and modulus, and a soft component, i.e., a polymer with relatively low glass transition temperature and modulus. In the past polyurethane polymers have been widely used as the soft component, while vinyl chloride or vinylidene chloride copolymers have been widely used as the hard component.
Magnetic pigments tend to agglomerate and can be difficult to initially disperse in the polymeric binder or be difficult to keep dispersed in the polymeric binder over time. Low molecular weight wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loadings, i.e., the use of greater amounts by weight of magnetic pigment, greater amounts of wetting agent or dispersant may be required. This is not always desirable since dispersants tend to plasticize binder systems and decrease their modulus. Further, excess dispersant may exude from a cured binder system over time, leading to changes in the properties of the media as well as to contamination of a recording head or the like.
To help alleviate the problems associated with low molecular weight dispersants or wetting agents, polymeric binders formed from "self-wetting" polymers have been developed. "Self-wetting" polymers have dispersing groups pendant from the polymer backbone that help disperse the magnetic pigment. Representative examples of dispersing groups include quaternary ammonium, amines, heterocyclic moieties, salts or acids based on sulfate, salts or acids based on sulfonate, salts or acids based on phosphate, salts or acids based on phosphonate, salts or acids based on carboxyl, mixtures thereof, and the like. As a result of using self-wetting polymers, little, if any, low molecular weight dispersant or wetting agent may be needed to further help disperse the pigments in the polymeric binder.
Polymers with quaternary ammonium groups that are useful as binders have generally been made either by incorporating the amine structure into the polymer backbone and subsequently quaternizing it or by incorporating the ammonium structure directly into the polymer backbone. Neither method allows for much flexibility and in both cases solubility and viscosity become critical issues.